In well cementing, such as well construction and remedial cementing, cement compositions are commonly utilized. Cement compositions may be used in a variety of subterranean applications. For example, in subterranean well construction, a pipe string (e.g., casing, liners, expandable tubulars, etc.) may be run into a well bore and cemented in place. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing method, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition may set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement (i.e., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the subterranean formation. Among other things, the cement sheath surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, for example, to seal cracks or holes in pipe strings or cement sheaths, to seal highly permeable formation zones or fractures, to place a cement plug, and the like.
A particular challenge in well cementing is the development of satisfactory mechanical properties in a cement composition within a reasonable time period after placement in the subterranean formation. During the life of a well, the cement sheath undergoes numerous strains and stresses as a result of temperature effects, pressure effects, and impact effects. The ability to withstand these strains and stresses is directly related to the mechanical properties of the settable composition after setting. The mechanical properties are often characterized using parameters such as compressive strength, tensile strength, Young's Modulus, Poisson's Ratio, elasticity, and the like. These properties may be modified by the inclusion of additives.
Conventional cement compositions have the limitation of shrinking during cement hydration. The shrinkage of the cement composition may result in the stresses that lead to damage of the cement sheath. The cement sheath may de-bond from the casing or formation resulting in micro-annuli and compromised zonal isolation. In some instances, such as certain combinations of depth and formation properties, even when external fluid is available to fully hydrate the cement composition, the cement sheath may become stressed during hydration and may not be able to withstand subsequent well operations.
One additive used to enhance mechanical properties is an anti-shrink additive. The anti-shrink additive helps prevent premature failure of the cement sheath by inhibiting or preventing cracks from forming. In some instances, cracks and gaps formed may lead to the migration of gas and fluid within the well and loss of zonal isolation. There may be considerable expense involved to repair a well with a failed cement sheath. In some instances the damage may be extensive enough to where the well needs to be abandoned.